Method for making a stressed structure designed to be dissociated

ABSTRACT

The invention concerns a method of making a complex microelectronic structure by assembling two substrates through two respective linking surfaces, the structure being designed to be dissociated at a separation zone. The invention is characterized in that is consists, prior to assembly, in producing a state difference in the tangential stresses between the two surfaces to be assembled, said difference being selected so as to obtain in the assembled structure a predetermined stress state at the time of dissociation.

TECHNICAL FIELD AND PRIOR ART

The invention relates to a method of producing a complex microelectronicstructure by assembling two basic microelectronic structures, thiscomplex structure being intended to be dissociated. The microelectronicstructure concept must be understood hereinafter as includingoptoelectronic, microtechnological, nanotechnological and nanoelectronicstructures.

The technique of transferring a layer from an original substrate to atemporary or final target substrate is increasingly used inmicroelectronics. This technique has many applications, of which onlytwo will be mentioned here by way of illustrative and non-limitingexample. For example, it is used to produce SOI (silicon on insulator)substrates used in particular to produce fast components with low powerconsumption. It is also used to produce composite substrates that limitcosts by avoiding the use of costly bulk substrates. This is the casewith bulk silicon carbide substrates, for example.

One prior art method of transferring a thin layer from a sourcesubstrate to a target substrate is described in the document FR 2 681472 and its various improvements (hereby incorporated by reference). Itcomprises the following steps in particular:

creation by ionic implantation of a buried weakened region within thesource substrate delimiting within that substrate the thin layer to betransferred,

assembling the source substrate to the target substrate at the freesurface of the thin layer, and

applying thermal and/or mechanical energy to cause a fracture in thesource substrate in the weakened region.

A problem can arise if heat treatment is required to induce some or allof the fracture in the weakened region and the source and targetsubstrates feature materials with very different coefficients of thermalexpansion. This is the situation, for example, if it is required totransfer a film of silicon onto a fused silica substrate. Heat treatmentcan induce high internal stresses within the structure formed byassembling the two substrates, by virtue of the difference in theircoefficients of thermal expansion, and these high internal stresses maydamage the structure. These stresses can also cause damage at the momentof fracture proper, since at this time the structures immediately relaxwhen they are suddenly dissociated. There is therefore at this moment asudden jump in the stresses in each structure, i.e. the structure formedof the transferred thin layer attached to the target substrate and thestructure formed by the remainder of the source substrate. If itsmagnitude is too high, this jump can damage at least one of these twostructures.

To solve this problem, it would be necessary, at the fracturetemperature, to be able to monitor precisely the stresses within thestructure formed by assembling the two substrates, in order to maintainthem below an acceptable stress level or even to minimize them.

More generally, the problem is that of controlling the stresses within aheterostructure (i.e. a complex structure made by assembling at leasttwo different materials) at the moment of dissociation of theheterostructure when that dissociation necessitates a change oftemperature.

SUMMARY OF THE INVENTION

To solve the stated problem, the invention proposes a method ofproducing a complex structure by assembling two substrates at respectiveconnecting faces thereof, the structure being adapted to be dissociatedin a separation region, characterized in that, prior to assembly, atangential stress state difference is created between the two faces tobe assembled, this difference being selected to obtain a predeterminedstress state within the assembled structure at the moment ofdissociation.

The tangential stress state difference between the two faces to beassembled is advantageously selected to minimize the stresses in theseparation region at the moment of dissociation.

Thus the invention teaches the intentional generation of stresses in theassembled structure to enable it to compensate the stresses that willsubsequently be generated when the temperature is increased todissociate the structure.

It is to be noted that, in an entirely different context, the paper byD. Feijoo, I. Ong, K. Mitani, W. S. Yang, S. Yu and U. M. Gösele,“Prestressing of bonded wafers”, Proceedings of the 1st internationalsymposium on semiconductor wafer bonding, Science, Technology andapplications, Vol. 92-7, The Electrochemical Society (1992) page 230,proposes a method of generating internal stresses within a complexstructure with a view to improving the mechanical stability of thestructure.

To this end, two structures, in this instance two silicon wafers, arebonded by molecular adhesion under the standard conditions. The complexstructure formed in this way is then stressed by curving it by applyinga pointer to the center of the structure, which is fixed at itsperiphery. By curving the structure sufficiently, the bonding interfaceyields: the two wafers separate and are then rebonded immediately withthe new curvature that has been achieved. This separation/rebondingprocess may be carried out several times, depending on the bondingenergy at the interface and the force applied by means of the pointer.When the authors release the stress caused by the pointer, the complexstructure relaxes and stabilizes at a radius of curvature that dependson that obtained at the time of the last separation/rebonding of thecomplex structure stressed by the pointer. Internal stresses aretherefore generated within the complex structure.

However, the internal stresses generated within the structure are noteasily adjustable using the above technique because they are dependenton the relative values of the elastic deformation energy of thestructure and the bonding energy. Moreover, as the authors indicate, theabove method cannot be used for molecular bonding at too high an energysince, under such conditions, the assembled two structures do notseparate and, if the pointer is removed within the range of elasticdeformation of the structure, the latter reverts to its initial state atthe time of molecular bonding. The structure therefore has no curvatureand therefore no internal stresses. Now, it is often technologicallybeneficial to have a high bonding energy, for example to ensure goodsolidity and a bonding interface of good quality.

The above document is nowhere concerned with monitoring the stresseswithin the structure linked to a change of temperature.

The technique described in the above document can undoubtedly be used togenerate stresses in a complex structure, but this idea is notassociated with solving the problem of a heterostructure's temperaturebehavior. It is therefore only a posteriori that the above documentmight be seen to have analogies with the invention. In any event, a lackof control of the stresses makes the proposed technique difficult toadjust. It is also limited to structures assembled with limited bondingenergies.

The method of the invention does not have the above limitations. Thestress state generated within the complex structure depends on stressesgenerated independently prior to assembly in each substrate. Thesestresses are accurately variable (see below). The method is thereforeaccurately reproducible and variable, enabling the stresses to bemonitored (or controlled) as a function of future requirements. Thebonding forces between the substrates to be assembled are no longerlimited since the complex structure does not have to be separated duringassembly by the method of the invention.

In the remainder of this document, the substrates to be assembled arealso referred to as basic structures, as opposed to the complexstructure formed by assembling these two substrates.

The tangential stress difference between the faces to be bonded of thetwo basic structures may advantageously be created by deforming (mainlyelastically) each of said structures before assembly. A simple and easytechnique for generating stresses is to curve these structures.

In a preferred embodiment, the two structures are curved so that the twofaces to be assembled are respectively concave and convex. They may becomplementary or even respectively spherical concave and sphericalconvex.

For example, the structures may be curved to generate stresses byapplying localized and/or distributed mechanical forces to thestructures to be deformed.

In a preferred embodiment, a pressure difference may be created betweenthe two faces of the structure to be curved. The means for producingthis pressure difference and for obtaining a basic structure having aface to be assembled include aspirating said structure onto a concavepreform of suitable profile selected as a function of that to beimparted to the face to be assembled and on which the structure restslocally at its periphery. Seals may advantageously be provided toimprove the seal between the structure and the preform. Aspiration ofsaid structure into a cavity (without perform) may also be mentioned,the structure resting locally at its periphery on a seal bordering thecavity.

The curved basic structure may be obtained by deforming the abovestructure between two complementary preforms, one concave and the otherconvex, with profiles selected as a function of that to be imparted tothe face to be assembled. In this case, aspiration channels may beprovided on the preform which receives the basic structure to keep thestructure curved, once the other preform has been removed. This otherpreform may advantageously be the other basic structure to be assembled,which has already been curved to the required profile.

Another option is to apply mechanical forces simultaneously to both ofthe structures to be assembled, for example by deforming the twostructures between two preforms with profiles selected as a function ofthose to be imparted to the faces to be assembled.

It is preferable if:

mechanical forces are applied to one or more of the substrates by meansof a preform consisting of a mold,

this preform consists of a porous mold, and

mechanical forces are applied to the substrates with the aid of at leastone deformable preform.

The two structures are preferably assembled by molecular bonding,achieving high adhesion forces and an interface of good quality. In thiscase, before or after creating the stress state difference between thetwo faces to be assembled, said faces are treated to facilitatesubsequent bonding. For example, the treatment may consist in mechanicaland/or chemical polishing, chemical treatment, UV/ozone treatment, RIE(reactive ion etching), plasma treatment, or annealing in hydrogen, etc.

According to other preferred features of the invention:

the substrates are assembled by direct contact, the surface of at leastone of the substrates being adapted to prevent air from being trappedbetween the assembled surfaces,

at least one of the substrates is pierced,

that substrate is pierced at its center,

at least one of the substrates includes at least one dead-end channeldischarging at the edge of the substrate,

the substrates are assembled by means of a flow layer,

assembly is carried out at a temperature above room temperature,

the substrates are heated by contact with heated preforms, and

the preforms are heated to respective different temperatures.

Also, the tangential stress state difference between the two faces to beassembled is advantageously selected so that the prestresses created inthis way within the complex structure enable subsequent imposition ofspecified internal stresses at a specified temperature. The prestressesare advantageously selected to minimize or eliminate the stresses withinthe complex structure.

The invention also provides a method for transferring a thin layer froma source substrate to a target substrate, comprising the followingsteps:

ionically implanting the source substrate through one face thereof tocreate a buried weakened layer at a particular depth relative to theimplanted face of the source substrate, a thin layer thereby beingdelimited between the implanted face and the buried layer,

assembling one face of the source substrate to one face of the targetsubstrate to form an assembled structure,

dissociating the thin layer from the remainder of the source substratein the buried layer,

the method being characterized in that, prior to assembly, a tangentialstress state difference is created between the two faces to beassembled, this difference being selected to obtain within the assembledstructure a predetermined stress state at the moment of dissociation.

The tangential stress state difference between the two faces to beassembled is advantageously selected to minimize the stresses in theburied layer at the moment of dissociation. This guarantees the qualityof the structures obtained after dissociation.

In a preferred embodiment, the tangential stress state differencebetween the two faces to be assembled is imposed by curving each of thetwo substrates to be assembled prior to assembly.

Other aspects and advantages of the invention will become apparent onreading the following detailed description of particular embodiments,which is given by way of non-limiting example. The description refers tothe appended drawings, in which:

FIG. 1 is a graph showing the evolution of the stresses with temperatureat the surfaces of the fused silica substrate within a conventional(silica +silicon) heterostructure,

FIG. 2 is a similar graph showing the stresses at the surfaces of thesilicon substrate of this heterostructure,

FIG. 3 is a diagram of a heterostructure obtained by the method of theinvention,

FIGS. 4 and 5 are graphs similar to those of FIGS. 1 and 2 showing theevolution of the stresses with temperature within a heterostructurestressed by the method of the invention,

FIG. 6 is a diagram of one non-limiting embodiment of the method of theinvention,

FIGS. 7, 8 and 9 show different ways of stressing basic structures to beassembled,

FIGS. 10A and 10B show from above two examples of producing one of thestructures to be assembled in such a way as to prevent trapping airbubbles, and

FIG. 11 is a diagrammatic view in section of a pair of deformablepreforms.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the figures to which the following description refers, identical,similar or equivalent parts are identified by the same referencenumbers. Also, to clarify the figures, the various items are notrepresented to a consistent scale.

To illustrate the invention, there will be described by way ofnon-limiting example a method of transferring a film consisting of alayer of silicon approximately 0.4 μm thick and a layer of oxideapproximately 0.4 μm thick from a 200 mm diameter surface-oxidizedsilicon source substrate 750 μm thick to a 200 mm diameter fused silicontarget substrate 1200 μm thick.

The film may be transferred using the following method, employingstandard transfer techniques:

ionically implanting the source substrate to create within thatsubstrate a weakened region that delimits the thin layer to betransferred under implantation conditions known to the person skilled inthe art, for example hydrogen implantation at a dosage of approximately6.10¹⁶ H⁺/cm² and at an energy of 75 keV,

bonding the oxidized layer of the source substrate to the targetsubstrate by molecular adhesion, and

transferring the thin film by fracture of the weakened region of thesource substrate, this fracture being induced by heat treatment atapproximately 400° C., for example, and advantageously being accompaniedby the application of mechanical forces.

FIGS. 1 and 2 respectively show the calculated stresses generated at therespective surfaces of the fused silica and silicon substrates duringheat treatment of the conventional complex structure formed byassembling the two substrates. At room temperature, the two substratesare relaxed and there is no internal stress within the complexstructure.

Thereafter, as the temperature rises, the structure is progressivelystressed: curve 1 in FIG. 1 shows the evolution of the stresses on theassembled face of the fused silica substrate, curve 2 shows theevolution of the stresses on its free face, curve 3 of FIG. 2 shows theevolution of the tensile stresses on the assembled face of the siliconsubstrate, and curve 4 shows the evolution of the tensile stresses onits free face.

This evolution of the stresses with temperature is perfectly familiar toand quantifiable by the person skilled in the art. It is described inthe following documents: S. Timoshenko, J. Opt. Soc. am. 11 (1925) page233 and D. Feijoo, I. Ong, K. Mitani, W. S. Yang, S. Yu and U. M.Gösele, Zhe-Chuan Feng and Hong-du Liu J; Appl. Phys. 54(1), 1983, page83 “Generalized formula for curvature radius and layer stresses causedby thermal strain in semiconductor multilayer structures”. To a firstapproximation, using continuous elastic theory mechanical calculations,if the materials are considered to be isotropic and the coefficients ofthermal expansion are considered to be constant over the applicabletemperature range, the evolution of the stresses is approximately linearwith temperature. More complex calculations (for example finite elementcalculations) may be used to refine these results.

Since silicon has a higher coefficient of thermal expansion than fusedsilica, as the temperature rises, expansion of the assembled face of thesilicon is impeded by the fused silica, which expands less than thesilicon. This face is therefore stressed in compression, causingexpansion of its free face because of its stiffness. In parallel withthis, the assembled face of the fused silica is expanded by the silicon,leading to compression of its free face because of the stiffness of thefused silica.

At the moment of fracture, these stresses are suddenly released. Thiscorresponds to a jump of approximately 100 MPa in the fracture region ofthe silicon substrate and of 160 MPa at the assembled face of the fusedsilica.

If they are not controlled, these stress jumps can damage the structuresobtained.

On the other hand, according to the invention, the structure isprestressed internally so that at the moment of fracture of theassembled complex structure the stresses are imposed and advantageouslybelow a threshold guaranteeing the quality of the structures obtainedafter fracture.

Consequently, if the silicon source substrate 5 and the fused silicatarget substrate 6 are curved prior to assembly with a radius ofcurvature at the faces to be assembled of the order of 1.2 m, and sothat the face to be assembled of the silicon substrate 5 is convex andthat of the fused silicon substrate 6 is concave, the structure shown inFIG. 3 is obtained after assembly with a given tangential stress levelat the bonding interface, the assembled face of the silicon beingstressed in tension at room temperature and that of the fused silicabeing stressed in compression at room temperature. The arrows in FIG. 3symbolize this stress state at the bonding interface. The line 5′symbolizes a weakened layer produced beforehand by implantation.

When stressed in this way, the stress level of the complex structureevolves with temperature in the manner shown in FIG. 4 for the assembledface (curve 7) of the fused silica and its free face (curve 8) and inFIG. 5 for the assembled face of the silicon (curve 9) and its free face(curve 10).

It may be seen that at the fracture temperature there is practically nointernal stress within the assembled complex structure, either in thesilicon substrate or in the fused silica substrate. There is thereforeno stress jump at the moment of dissociation.

In this example, the internal stresses in the fracture region havingbeen minimized at the fracture temperature, those stresses are no longeroperative in the fracture mechanism. This can have only a negligibleinfluence on the thermal budget (temperature-time pair) necessary forthe fracture and in this case the same thermal budget is retained. Incertain cases it could be necessary to modify the heat treatment timefor the fracture to be effected compared to the time necessary for thefracture in the case of bonding without prestressing, for example. If itis a requirement that the heat treatment time should not be modified, itwould equally be possible to change the heat treatment temperature. Aradius of curvature prior to bonding may then advantageously be selectedthat is adapted to this new temperature. It may also be beneficial toreduce the internal stresses in the assembled structure, withoutminimizing them. This is the case, for example, if it is necessary toselect the thermal budget necessary for the fracture while at the sametime guaranteeing the quality of the structures obtained after fracture.The choice may of course be made to apply either generalized orlocalized stresses (tension, torsion, etc.) of external origin.

Generating a tangential stress difference between the assembled faces ofthe source and target substrates therefore limits the stress jumpssuffered by the various structures obtained at the moment ofdissociation of the assembled structure. If the two substrates to beassembled are bulk substrates and have different coefficients of thermalexpansion, the tangential stress state difference between the two facesto be assembled is advantageously selected so that the face of thesubstrate having the lower coefficient of thermal expansion is stressedin compression relative to the other face to be assembled. The resultingprestressing of the complex structure will then compensate in advancesome or all of the future stresses, linked to the rise in temperature,in particular at the dissociation temperature.

Given the selected dissociation temperature and the coefficients ofexpansion of the materials involved, it will be obvious to the personskilled in the art how to determine the stresses to be generated at thetime of assembly.

There are many ways of generating this stress state difference.

FIG. 6 shows one embodiment of the method. A first basic structure 11 isdeformed by aspirating it onto a first preform 12 of specific shape, forexample of spherical concave shape. Aspiration is effected by means ofaspiration channels 15 opening onto the surface of the preform. Seals 16at the periphery of the preform support the first structure 11 andensure a pressure difference to be obtained between two faces of thatstructure. Because of this pressure difference, the structure isdeformed to espouse the shape of the first preform 12. Because of thisdeformation, stresses familiar to and quantifiable by the person skilledin the art are generated within the first structure 11 and in particularat its exposed face (here its upper face).

A second structure 13 is then offered up facing the exposed face of thefirst structure 11. A second preform 14 with a suitable shape that isadvantageously complementary to the shape of the first preform 12, forexample of spherical convex shape, is provided to elastically deform thesecond structure 13 between the second preform 14 and the firststructure 11. The arrow represented in FIG. 6 symbolizes the applicationof forces to effect the deformation proper. During the deformation, thesecond structure 13 is progressively deformed into contact with thefirst structure 11 until it espouses its shape.

The two faces to be assembled having been treated in a manner familiarto the person skilled in the art to enable bonding by molecularadhesion, for example prior to stressing the two structures, bonding isthen effected when the two faces coincide.

A complex structure of the type in FIG. 3 is then obtained formed by theassembly of two stressed structures having at their assembled faces aknown tangential stress difference that is imposed by the respectivedeformations of the two structures prior to bonding.

The person skilled in the art knows how to link the deformation imposedon the structures accurately to the stress levels obtained in thestructure and in particular those obtained on the faces to be assembled.He therefore knows, through an appropriate choice of the shapes of thepreforms 12 and 14, how to impose a precise tangential stress differencebetween the two faces to be assembled before bonding and thus to imposestresses throughout the complex structure once assembled. The preformsmay be rigid porous or non-porous molds, for example, or deformablemembranes.

As shown in FIG. 7, a variant of the method replaces the first preform12 with a hollow device 17 having a central cavity 18. The periphery ofthe first structure 11 then rests on this device with seals 19sandwiched between them. Aspiration channels 20 reduce the pressureinside the cavity. Adjusting the pressure difference between the twofaces of the first structure 11 deforms the first structure 11 to aparticular curvature. For example, for a vacuum in the cavity ofapproximately 0.25 bar, the other face of the structure being exposed toatmospheric pressure, a deflection of 3 mm is obtained in the case of astandard 200 mm diameter silicon wafer 750 μm thick using a seal of 195mm diameter. The first structure 11 can then be assembled to the secondstructure 3 in the manner explained above.

FIG. 8 shows another variant which consists in deforming the secondstructure 13 between two appropriate preforms of complementary shape,one (22) concave and the other (21) convex. The convex preform isprovided with aspiration channels 24 for holding the second structure 13in position after deformation and removal of the concave preform 22. Thesecond structure 13 may then be assembled to the first structure 11,which itself has already been deformed (for example in accordance withFIG. 7), by bonding with the aid of an adhesive, for example.

Another variant assembles the two basic structures by molecular bondingat room temperature and without stress. The assembled structure is thendeformed between two complementary molds. After verifying that each ofthe structures is fastened to one of the molds (for example byaspiration), the assembled structure is separated from the molecularbonding area by any means known to the person skilled in the art. Thisyields two stressed basic structures that may thereafter be assembled inaccordance with the invention. This variant has the advantage ofpreserving the surface state of the faces to be assembled, for exampleenabling assembly of the two stressed basic structures by furthermolecular bonding.

The assembly of the two basic structures can therefore be effected bymolecular adhesion, by bonding by means of an adhesive, or by bonding bymeans of a bedding layer.

An adhesion layer may be used between the preform and the structure tobe deformed or electrostatic or magnetic forces may be used to hold thepreform and the curved basic structure in contact.

In a further variant, shown in FIG. 9, the two structures 11 and 13 areplaced face to face without bonding them and are deformed simultaneouslybetween a concave preform 25 and a convex preform 26 with complementaryshapes. In FIG. 9 the arrows show the pressure forces to be applied tocause the deformation. The two structures are then deformed conjointly,a film of air remaining between the two structures. Once the requiredcurvature has been achieved, the air film is evacuated and, because ofthe forces applied, bonding by molecular adhesion then takes place.

When the second structure 13 is deformed between the first structure 11and the preform 14, an air bubble may be trapped between the twostructures and impede bonding by molecular adhesion. To evacuate thisair bubble, it is advantageous to pierce one or both of the structuresto be assembled at their center 27, as shown in FIG. 10A, for example bylaser drilling or deep etching of the structure.

An alternative is to provide on one or both structures one or moreevacuation channels 28 on the face to be assembled and discharging atthe edge of the wafer, as shown in FIG. 10B. For example, these channelsmay have dimensions of the order of a width of 100 μm and a depth of 5μm and be produced by the usual lithography and etching techniques.Aspiration means could be associated with these channels 28 or thepiercing 27 to facilitate evacuation of the trapped air.

Another option may be to effect the deformation and the assembly under apartial vacuum in order to minimize the volume of trapped air, althoughthis method has the drawback of necessitating much harder vacuums toensure deformation of the structures.

A final option entails placing radial spacers at the wafer periphery andremoving them once the central area has been bonded. More generally, anymethod may be used that enables initiation of bonding between the twostructures at their center which then propagates toward the edges. Forexample, a slight difference in radius of curvature between the twostructures could be introduced before bonding to achieve this.

After bonding by the method of the invention, there is obtained, byimposing a tangential stress difference between the phases of the twostructures to be assembled, a stressed complex structure in which thestresses at all points are known. When the forces that deformed the twoinitial structures (mechanical pressure or aspiration by means of avacuum) are removed, upon the release of the exterior faces of thecomplex structure, the stresses within that structure evolve, but in aparticular manner that is known to the person skilled in the art. Amongother things, this evolution is a function of the natures and thethicknesses of the various materials constituting each of the twoinitial structures and the stress difference at the bonding interface.

The methods described above enable dissociation under controlledstresses of a heterostructure formed of substrates of differentmaterials. These substrates may be thicker or thinner, of simple orcomposite form (formed of a stack of different layers of thicker orthinner materials), processed or not. The materials concerned are allthe semiconductors, such as in particular silicon, germanium, theiralloys (Si_(1-x)Ge_(x)), indium phosphide (InP), gallium arsenide(GaAs), lithium niobate, silicon carbide (SiC), gallium nitride (GaN),sapphire, superconductors such as compositions of the YbaCuO, NbN, orBiSrCaCuO type, for example, or insulators such as, in particular, fusedsilica, quartz, glasses with different compositions, MgO, all metalssuch as in particular tungsten, copper or aluminum.

Diverse variants of the foregoing are feasible.

The preforms may be heated to enable hot bonding of deformedintermediate structures. The preforms may advantageously be at differenttemperatures so that the two intermediate structures have a temperaturedifference at the moment of assembly.

Bonding the intermediate structures at high temperature also providescontrol over the internal stresses of the complex structure, in additionto the control already achieved through the controlled deformation ofthe intermediate structures.

For example, it is then possible to cancel the internal stresses of acomplex structure at a given temperature by limiting the deformation ofthe intermediate structures. For example, it is not wished to deform toa radius of curvature of more than 1.4 m the two intermediate structuresconsisting of a 750 μm thick silicon wafer 200 mm in diameter and a 1200μm thick fused silica wafer 200 mm in diameter. These two intermediatestructures, deformed to a radius of curvature of approximately 1.4 mprior to bonding, yield a complex structure in which the internalstresses are eliminated at approximately 300° C. if bonding took placeat 20° C. On the other hand, if the two intermediate structures arebonded at 100° C., the internal stresses of the complex structure areeliminated at 380° C., and thus at a higher temperature without havingto deform further the intermediate structures.

A layer that flows at a certain temperature Tf may be placed between thetwo intermediate structures. Introducing this flow layer modifies theinternal stresses in the complex structure if the heat treatmenttemperature exceeds Tf.

This minimizes stresses during annealing, for example. Consider, by wayof example, a complex structure consisting of a 1200 μm thick fusedsilica substrate of 200 mm diameter on which there is a 0.4 μm thickfilm of silicon. Creating the complex structure by means of theinvention means that a heat treatment temperature Ttth of 800° C., forexample, can be achieved without exceeding the stress level set topreserve good crystal quality in the silicon film (without prestressingthe basic structures to form the complex structure, a temperature of800° C. could not be reached without degrading the silicon film). On theother hand, if it is required to raise the heat treatment temperaturewithout modifying the deformation of the basic structures used to obtainthe complex structure, there is then the risk of exceeding the stresslevel that has been set. If there is a layer that flows at Tf, with Tfequal to 800° C., for example, as soon as the heat treatment temperatureexceeds Tf the flow layer will flow, thereby relieving some of theinternal stresses. Heat treatment can then be carried out at atemperature higher than Ttth without exceeding the internal stress levelthat has been set.

The preforms may be molds, for example porous molds.

If a pressure difference is used to deform the basic structures or toretain the basic structures on the preforms, it can be advantageous forone of the faces of the basic structures to be at a pressure other thanatmospheric pressure, advantageously a pressure higher than atmosphericpressure. FIG. 11 shows by way of example an enclosure 30 containing twopreforms 31 and 32 each including a deformable membrane 31A or 32A.Aspiration channels 33 and 34 open onto the surface of these membranes,and are represented here as being tangential. The aspiration orpressurization circuits are represented by double lines.

The aspiration channels maintain the basic structures in their deformedstate; the area of the aspiration channels may be limited by subjectingthe exposed face of the intermediate structure to a pressure higher thanatmospheric pressure (for example a pressure inside the enclosure of 2bar). Moreover, if the deformable preform is deformed by a pressuredifference, a greater deformation can be achieved by increasing thepressure on the exposed face of the basic structure. For example, thepreform 31 is at an internal pressure of 1.5 bar, the channels 33 are ata pressure of 0.3 bar, the preform 32 is at an internal pressure of 2.5bar, and the channels 34 are at a pressure of 0.3 bar. The pressure ofthe enclosure (2 bar) is between the pressures of the preforms 31 and32.

1. A method of producing a complex structure, the structure beingadapted to be dissociated in a separation region, the method comprisingassembling two substrates at respective connecting faces thereof,wherein prior to assembly, a tangential stress state difference iscreated between the connecting faces by applying mechanical forces tocurve each of the two substrates wherein the tangential stress statedifference is selected to produce a predetermined stress state withinthe complex structure at the moment of dissociation.
 2. The methodaccording to claim 1, wherein the tangential stress state differencebetween the connecting faces is selected to minimize the stresses in theseparation region at the moment of dissociation.
 3. The method accordingto claim 1, further comprising curving the two substrates curved so thatthe connecting faces comprise respectively concave and convex faces. 4.The method according to claim 3, wherein curving the two substratescomprises curving so that the connecting faces comprise complementaryfaces.
 5. The method according to claim 4, wherein curving the twosubstrates comprises curving so that the connecting faces compriserespectively spherical concave and spherical convex faces.
 6. The methodaccording to claim 1 wherein applying mechanical forces comprisescreating a pressure difference between the connecting faces.
 7. Themethod according to claim 6, wherein creating a pressure differencebetween the connecting faces comprises aspirating one of the twosubstrates onto a concave preform having a suitable profile andimparting the profile to a face of the one substrate, and wherein theone substrate rests on the concave preform at its periphery.
 8. Themethod according to claim 6, wherein creating the pressure differencebetween the connecting faces comprises aspirating one of the twosubstrates into a cavity, the one substrate resting locally at itsperiphery on a seal bordering the cavity.
 9. The method according toclaim 1 wherein applying mechanical forces comprises deforming one ofthe two substrates between complementary first and second preforms, oneof which is concave and the other of which is convex, and impartingselected profiles to the connecting face.
 10. The method according toclaim 9, wherein the first complementary preform comprises the other ofthe two wherein the substrate is curved to have a selected profile. 11.The method according to claim 9, wherein the second preform includesaspiration channels for keeping the one substrate curved when the firstpreform has been removed.
 12. The method according to claim 1, whereinapplying mechanical forces comprises applying mechanical forcessimultaneously to the two substrates by deforming the two substratesbetween two preforms having selected profiles to be imparted to theconnecting faces.
 13. The method according to claim 1, wherein applyingmechanical forces comprises applying mechanical forces to at least oneof the substrates by means of a preform comprising a mold.
 14. Themethod according to claim 13, wherein the preform comprises a porousmold.
 15. The method according to claim 1, wherein applying mechanicalforces comprises applying mechanical forces to the two substrates usingat least one deformable preform.
 16. The method according to claim 1,wherein assembling the two substrates comprises molecular bonding. 17.The method according to claim 1 further comprising treating theconnecting faces to facilitate bonding.
 18. The method according toclaim 1, wherein the two substrates are assembled by direct contact,wherein the face of at least one of the two substrates is adapted toprevent air from being trapped between the connecting faces.
 19. Themethod according to claim 18 further comprising piercing at least one ofthe two substrates.
 20. The method according to claim 19, whereinpiercing at least one of the two substrates comprises piercing thesubstrate at its center.
 21. The method according to claim 18 furthercomprising forming in at least one of the two substrates at least onedead-end channel discharging at the edge of the substrate.
 22. Themethod according to claim 1, wherein the two substrates are assembled bymeans of a flow layer.
 23. The method according to claim 1 wherein thetwo substrates are assembled at a temperature higher than roomtemperature.
 24. The method according to claim 23 further comprisingheating the two substrates by contact with heated preforms.
 25. A methodaccording to claim 24, wherein the preforms are heated to respectivedifferent temperatures.
 26. A method for transferring a thin layer froma source substrate to a target substrate comprising the following steps:ionically implanting the source substrate through a face thereof tocreate a buried weakened layer at a particular depth relative to theface of the source substrate, a thin layer thereby being delimitedbetween the face and the buried weakened layers; curving each of thesource substrate and the target substrate by applying mechanical forcesto create a tangential stress state difference between the face of thesource substrate and a face of the target substrate; assembling the faceof the source substrate to the face of the target substrate to form anassembled structure; and dissociating the thin layer from a remainder ofthe source substrate, wherein the tangential stress state difference isselected to produce a predetermined stress state within the assembledstructure at the moment of dissociation.
 27. The method according toclaim 26, wherein creating a tangential stress state difference betweenthe faces comprises creating a tangential stress state difference tominimize internal stresses at the moment of dissociation.